Influent Cr Research Articles

Hexavalent Chromium Reduction by Bacillus sp. in a Packed-Bed Bioreactor

The potential for fixed-film biorectors to reduce Cr(VI) was demonstrated using a Cr(VI)-reducing species, Bacillus sp. A bench-scale, packed-bed bioreactor was operated to steady-state conditions under a range of influent Cr(VI) concentrations (10−200 mg/L) and hydraulic detention times (6−24 h, clean bed) with near complete removal of Cr(VI). The steady-state Cr(VI) reduction efficiency was not affected by the influent Cr(VI) concentration or hydraulic detention time. Chromium mass balance analysis revealed that nearly all the Cr(VI) fed to the bioreactor was accounted for in the effluent as Cr(VI) and Cr(III). Total cell mass in the bioreactor decreased with increasing Cr(VI) loading rate, but it stabilized after a loading limit was reached (1016 mg of Cr(VI)/L·day) when operated under 24 h hydraulic detention time. The bioreactor showed strong resilience by recovering from Cr(VI) overloading through reduction in influent Cr(VI) concentration.

Hexavalent Chromium Removal in Two-Stage Bioreactor System

A two-stage bioreactor system was used to reduce Cr(VI). Escherichia coli ATCC 33456 cells were aerobically grown in a first-stage chemostat, and then pumped to a second-stage plug-flow reactor where anaerobic Cr(VI) reduction occurred. Experimental results demonstrated that near complete removal of Cr(VI) was achieved in the plug-flow reactor under appropriate operating conditions. The removal efficiency in the plug-flow reactor was significantly affected by the influent Cr(VI) concentration, Cr(VI) loading rate, liquid detention time, and a consumed Cr(VI) reduction capacity factor. The consumed Cr(VI) reduction capacity factor was directly related to the toxicity effect of Cr(VI) on cells. To illustrate the influence of the consumed reduction capacity factor on Cr(VI) reduction in the plug-flow reactor, a mathematical model was developed by incorporating Cr(VI) reduction kinetics into the mass balance relationship of Cr(VI) for a plug-flow reactor. Analyses of Cr(VI) concentration profiles along the plug-flow reactor using both model simulations and experimental data indicated that the rate of Cr(VI) reduction decreased with the depletion of the reduction capacity of cells and Cr(VI) reduction ceased after the reduction capacity was exhausted.

Studies on microbial chromate reduction by Pseudomonas Sp. In aerobic continuous suspended growth cultures

Reduction of hexavalent chromium was studied in three bench-scale continuous stirred tank reactors. The inoculum was a culture of Pseudomonas sp., capable of giving 83% to 87% chromate reduction in 72-h batch assays with 60 mg Cr(VI) L(-1) in synthetic medium. The continuous culture studies were conducted for about 100 days using synthetic feed containing different levels of chromate (5 to 124 mg L(-1)) at 28 degrees to 30 degrees C and pH 6.8. The feed rate was varied over the range 0.5 to 1 L d(-1) to obtain hydraulic retention time of 36 to 72 h. Chromate reduction efficiency was 81% to 91% and 100% for influent Cr(VI) concentrations of 15 to 124 and 5 mg L(-1), respectively, with a hydraulic retention time of 72 h. (c) 1994 John Wiley & Sons, Inc.

Reduction of hexavalent chromium by Pseudomonas fluorescens LB300 in batch and continuous cultures

Batch and continuous cultures of Pseudomonas fluorescens LB300 were shown to reduce hexavalent chromium, Cr(VI), aerobically at neutral pH (pH 7.0) with citrate as carbon and energy source. The product of Cr(VI) reduction was previously shown and confirmed in this work to be trivalent chromium, Cr(III), by quantitative reoxidation to Cr(VI) with KMnO4. In separate batch cultures (100 ml) containing initial Cr(VI) concentrations of 314.0, 200.0 and 112.5 mg Cr(VI) L−1, the organism reduced 61%, 69% and 99.7% of the Cr(VI), respectively. In a comparison of stationary and shaken cultures, the organism reduced 81% of Cr(VI) in 147 h in stationary culture and 80% in 122 h in shaken culture. In continuous culture, the organism lowered the influent Cr(VI) concentration by 28% with an 11.7-h residence time, by 39% with a 20.8-h residence time and by 57% with a 38.5-h residence time. A mass balance of chromium in a continuous culture at steady state showed an insignificant uptake of chromium by cells of P. fluorescens LB300.

More on Mechanism and Some Important Properties of Chromate Ion Exchange

Sufficient experimental data are presented here to verify the chromate ion‐exchange mechanism proposed earlier by Sengupta and Clifford (1986a). Most of the experiments were tailored to prove the validity of the mechanism, and also to highlight the unusual effects associated with it. One significant finding of this study is that, in spite of very gradual breakthrough during fixed‐bed column runs, chromate (VI) concentration at the exit of the column remains almost independent of the influent Cr(VI) concentration for a long time after the start of the run. Mass‐transfer limited kinetics are not responsible for this conspicuous column characteristic. Such an anomalous behavior is explained with the proposed ion‐exchange mechanism, which suggests dimerization of HCrO4- into Cr2O72- inside the commercial organic‐base anionexchange resins. A more theoretically sound attempt can now be developed for designing fixed bed chromate‐exchange processes at acidic pH in the presence of other competing ions.